Non-aqueous electrolyte secondary battery

ABSTRACT

Disclosed is a negative electrode active material offering a long life non-aqueous electrolyte secondary battery with high energy density that shows excellent cycle life characteristics. The negative electrode active material comprises a compound represented by the formula DSnO3 wherein D represents at least one selected from the group consisting of alkaline earth metals.

CROSS-REFERENCE TO RELATED APPLICATIONS

This is a continuation-in-part application of application Ser. No.09/080,986, filed May 19, 1998, now abandoned.

BACKGROUND OF THE INVENTION

The present invention relates to a non-aqueous electrolyte secondarybattery, particularly an improvement of a negative electrode usedtherefor.

There have been various vigorous studies on a non-aqueous electrolytesecondary battery including lithium or a lithium compound as a negativeelectrode, because it is to be expected to offer a high voltage as wellas a high energy density.

To date, oxides and chalcogens of transition metals like LiMn₂O₄,LiCoO₂, LiNiO₂, V₂O₅, Cr₂O₅, MnO₂, TiS₂, MoS₂ and the like are knownpositive electrode active materials for non-aqueous electrolytesecondary batteries. Those compounds have a layered or tunneled crystalstructure that allows free intercalation and deintercalation of lithiumions. On the other hand, there are many previous studies on metalliclithium as the negative electrode active material. However, metalliclithium has a drawback that when used as the negative electrode activematerial, a deposition of lithium dendrites occurs on the surface oflithium during charging, which reduces charge/discharge efficiency orcauses internal short-circuiting due to contact between formed lithiumdendrites and the positive electrode. As one measure for solving suchdrawback, the use of a lithium alloy such as lithium-aluminum alloywhich not only suppresses the growth of lithium dendrites but also canabsorb therein and desorb therefrom lithium as the negative electrodehas been under investigation. However, the use of such lithium alloy hasa drawback that repeated charge/discharge operation causes pulverizationof the alloy as the electrode, which in turn deteriorates the cycle lifecharacteristics of a battery.

Therefore, there are proposals to suppress pulverization of theelectrode by using a lithium-aluminum alloy including additionalelements as electrode (e.g., Japanese Laid-Open Patent Publications Sho62-119856 and Hei 4-109562). Under the circumstance, however, theimprovement remains unsatisfactory. At present, lithium ion batterieshave been put into practical use that include as the negative electrodea highly safe carbon material capable of reversibly absorbing anddesorbing lithium and having exceptional cycle life characteristicsalthough smaller in capacity than the above-mentioned negative electrodeactive materials. In an attempt to realize a higher capacity, variousproposals have been made of an application of oxides to negativeelectrodes. For example, it is suggested in Japanese Laid-Open PatentPublications Hei 7-122274 and Hei 7-235293 that crystalline oxides suchas SnO and SnO₂ may serve as negative electrode active materials withhigher capacities than the conventional oxide WO₂. There is anotherproposal in Japanese Laid-Open Patent Publication Hei 7-288123 to usenon-crystalline oxides such as SnSiO₃ or SnSi_(1−x)P_(x)O₃ for thenegative electrode in order to improve the cycle life characteristics.But, the improvement is still unsatisfactory.

BRIEF SUMMARY OF THE INVENTION

The primary object of the present invention is to provide a negativeelectrode for non-aqueous electrolyte secondary batteries havingexcellent charge/discharge cycle life characteristics.

Another object of the present invention is to provide a negativeelectrode affording a high electric capacity and an exceptional cyclelife by absorbing lithium upon charging without growing lithiumdendrites.

The present invention provides a non-aqueous electrolyte secondarybattery comprising a chargeable and dischargeable positive electrode, anon-aqueous electrolyte and a chargeable and dischargeable negativeelectrode, wherein the negative electrode comprises a compoundrepresented by the formula (1)

DSnO₃  (1)

wherein D represents at least one alkaline earth metal.

In a preferred mode of the present invention, D is represented by theformula (2)

Sr_(x)Ba_(1−x)  (2)

wherein 0.03≦x≦0.5.

It is further preferable that the range of x is 0.1≦x≦0.5 in the formula(2)

While the novel features of the invention are set forth particularly inthe appended claims, the invention, both as to organization and content,will be better understood and appreciated, along with other objects andfeatures thereof, from the following detailed description taken inconjunction with the drawings.

BRIEF DESCRIPTION OF SEVERAL VIEWS OF THE DRAWING

FIG. 1 is a brief longitudinal cross-sectional view of a test cell forevaluating the characteristics of an electrode of an active material inaccordance with the present invention.

FIG. 2 is a longitudinal cross-sectional view of a cylindrical batteryused for embodying the present invention.

FIG. 3 is an X-ray diffraction pattern A and B obtained from a negativeelectrode active material of MgSnO₃ and SnO₂ respectively of a test cellupon cathode polarization at the 10th cycle.

FIG. 4 is a description showing x-ray diffraction patterns of BaSnO₃,SrSnO₃, CaSnO₃ and MgSnO₃ at the first charged state.

FIG. 5 is a description showing x-ray diffraction patterns of BaSnO₃ andSr_(x)Ba_(1−x)SnO₃.

FIG. 6 is a description showing the relation between the dischargecapacity and cycle number of a test cell using BaSnO₃, SrSnO₃, CaSnO₃,MgSnO₃ and Sr_(0.1)Ba_(0.9)SnO₃ for an active material

DETAILED DESCRIPTION OF THE INVENTION

Upon being integrated in a battery, lithium is intercalated into thenegative electrode active material in accordance with the presentinvention during charging in normal cases. In the formula Li_(θ)DSnO_(γ)representing the composition of a composite compound intercalated withlithium, the content of lithium represented by θ is preferably in arange of 1≦θ<10. When 10≦θ, the compound can give only poor cycle lifecharacteristics and thus not practical. On the other hand, when θ<1, thecompound disadvantageously produces a small capacity. It is assumed thatwhen such compound already intercalated with lithium is subjected torepeated intercalation and deintercalation of lithium bycharge/discharge cycles, the compound loses its original compositionpartially. Therefore, it seems appropriate that the compound has acomposition where the respective elements Li, D, Sn and 0 are present inan atomic ratio of θ:1:1:γ

According to the present invention, a highly reliable non-aqueouselectrolyte secondary battery with a high energy density as well as anexceptional cycle life which is free from development ofdendrite-induced short-circuiting.

In the following, the present invention will be described referring tospecific examples, although the present invention is not limitedthereto.

EXAMPLE 1

In the present example, compounds represented by the formula (1) wereevaluated.

First, for evaluation of the electrode characteristics of those compoundas negative electrode active materials, test cells as shown in FIG. 1were fabricated.

A mixture was prepared by mixing 3 g of a graphite powder as aconductive agent and 1 g of a polyethylene powder as a binder with 6 gof each active material powder. Then, 0.1 g of the mixture waspressure-molded to a disc of 17.5 mm in diameter. An electrode 1 thusformed was. placed in the center of a case 2 and disposed thereon with aseparator 3 of a microporous polypropylene film.

A non-aqueous electrolyte prepared by dissolving 1 mol/l lithiumperchlorate (LiClO₄) in a mixed solvent of ethylene carbonate anddimethoxyethane in a volumetric ratio of 1:1 was injected over theseparator 3. Next, the case 2 was combined with a sealing plate 6 havinga polypropylene gasket 5 on the periphery thereof and attached with ametallic lithium disc 4 of 17.5 mm in diameter to the inner surfacethereof, then was sealed to complete a test cell.

Each of the test cells thus produced was subjected to cathodepolarization (corresponding to charging when the active materialelectrode is taken as the negative electrode) at a constant current of 2mA until the electrode potential became 0 v vs. lithium counterelectrode. Then, the test cell was subjected to anode polarization(corresponding to discharging) until the electrode potential dropped to1.5 V vs. lithium counter electrode. Cathode and anode polarizationswere repeated and the electrode characteristics were evaluated in allthe test cells.

For comparison, conventional oxides and sulfides of metals listed inTable 1 were used.

The present example used those oxides listed in Table 2.

The discharge capacities per gram of the active material at the 1stcycle in the test cells are summarized in Tables 1 and 2, respectively.

All of the test cells including oxides of Example 1 in accordance withthe present invention were found chargeable and dischargeable. Uponcompletion of cathode polarization at the 10th cycle, the test cellswere disassembled and found to have no deposits of metallic lithium.

The above results indicated that the electrodes including the activematerials in accordance with the present invention absorb thereinlithium upon cathode polarization and desorb therefrom absorbed lithiumupon anode polarization without growing dendrites of metallic lithium.

Next, for evaluation of the cycle life characteristics of the batteriesapplied with the negative electrodes of the active materials inaccordance with the present invention, cylindrical batteries as shown inFIG. 2 were produced.

The batteries were produced as follows:

First, a positive electrode active material LiMn_(1.8), Co_(0.2)O₄ wassynthesized by mixing Li₂CO₃, Mn₃O₄ and CoCO₃ in a predetermined molarratio, followed by heating at 900° C. The resultant was further filteredthrough 100 mesh or less before used as the positive electrode activematerial of Example 1.

Then, to 100 g of the positive electrode active material, 10 g of acarbon powder as a conductive agent, 8 g (in solids) of an aqueousdispersion of polytetrafluoroethylene as a binder and pure water wereadded to form a paste. The paste was applied on a titanium corematerial, dried and rolled. In this way, a positive electrode plate wasproduced.

Separately, a negative electrode plate was prepared as follows:

Each of various active materials, a graphite powder as a conductiveagent and polytetrafluoroethylene as a binder were mixed in a weightratio of 60:30:10 and the mixture was made into a paste using apetroleum solvent. The paste was applied on a copper core material,followed by drying at 100° C. to form a negative electrode plate.

A porous polypropylene film was used as the separator.

Then, a positive electrode plate 11 having a spot-welded positiveelectrode lead 14 made of the same material as that of the core materialwas combined with a negative electrode plate 12 similarly having aspot-welded negative electrode lead 15 made of the same material as thatof the core material together with a band-like separator 13 of a porouspolypropylene film interposed therebetween and the combination wasspirally rolled up to make an electrode group. The electrode group wasplaced in a battery case 18 after adhering polypropylene insulatingplates 16 and 17 to the top and the bottom of the electrode group. Astep was formed at the upper part of the battery case 18 and anon-aqueous electrolyte prepared by dissolving 1 mol/l lithiumperchlorate in a mixed solvent of ethylene carbonate and dimethoxyethanein an equivolumetric ratio was injected into the battery case 18. Then,the case was sealed using a sealing plate 19 provided with a positiveterminal 20 to form a battery.

Each of the batteries thus formed was subjected to a charge/dischargecycle test under conditions of a temperature of 30° C., acharge/discharge current of 1 mA/cm² and a charge/discharge voltage in arange of 4.3 to 2.6 V.

Tables 1 and 2 summarize the discharge capacity maintenance ratios after100 cycles in the batteries including negative electrodes of the oxidesof the comparative example and Example 1, using their dischargecapacities at the 2nd cycle as reference.

TABLE 1 Capacity Capacity Comparative Example (mAh/g) maintenance rate(%) WO₂ 190 9 Fe₂O₃ 185 10 SnO 522 5 SnSiO₃ 453 20 PbO 453 2 SnS 498 6PbS 436 3 SnSi_(0.8)P_(0.2)O_(3.1) 406 25

TABLE 2 Capacity Capacity Example (mAh/g) maintenance rate (%) MgSnO₃550 85 CaSnO₃ 570 90 SrSnO₃ 630 95 BaSnO₃ 400 95 Sr_(0.1)Ba_(0.9)SnO₃630 95 Sr_(0.3)Ba_(0.3)SnO₃ 620 95 Sr_(0.5)Ba_(0.5)SnO₃ 600 95

As is evident from the tables, the batteries using the negativeelectrodes of the oxide active materials in accordance with the presentinvention are improved drastically in the cycle life characteristics ascompared with those using the negative electrodes of the conventionaloxides.

Next, the factor contributing to the improved cycle life characteristicsof the above-mentioned active materials of the present invention wasanalyzed. FIG. 3 shows an X-ray diffraction pattern A obtained from thetest cell using MgSnO₃ as the negative electrode active material uponcompletion of cathode polarization (charged state of the negativeelectrode active material) at the 10th cycle. The figure also lists anX-ray diffraction B in the test cell using the comparative exampleactive material SnO₂. Noting the peak around 2θ=38°, a sharp peakclearly indicating the presence of an Li—Sn alloy was observed in thecomparative example oxide. On the other hand, a very broad peak with alow peak intensity was observed in the oxide of Example 1.

The above findings suggested that the charge/discharge reaction in thecomparative example oxide SnO₂ develops basically by the alloyingreaction between Sn and Li. In the oxide MgSnO₃ of Example 1, althoughit was speculated that this oxide also experiences the same reaction, itshows a broad peak with very low peak intensity on the X-ray diffractionpattern as compared with the comparative example. This suggests very lowcrystallinity of the Li—Sn alloy synthesized during charge of MgSnO₃ ascompared with the comparative example oxide SnO₂. Although the detailsremain to be clarified more, the low crystallinity was considered toresult from the prevention by the presence of the group D element Mg ofa reduction of the reactive surface area or inactivation due to anaggregation of Sn. This seems to have led to improved cycle lifecharacteristics.

Although only MgSnO₃ was exemplified above, the same observations wereobtained from the rest of the active materials.

The compound used as the active material in the present invention isgenerally synthesized in the state where Li is not present. In thecrystal structure of the above compound, Sn is positioned at regularintervals around the alkaline earth metal element and oxygen ispositioned between Sn and Sn. Consequently, it is considered that abattery in which Sn is not liable to aggregate even after repeatingcharge/discharge reaction and having a long cycle life can be obtained.It is also considered that the greater the ionic diameter of thealkaline earth metal, the more effectively the aggregation of Sn can beinhibited. Further, it is considered that BaSnO₃ is extremely stable inrepeating the charge/discharge reaction because the ionic diameter of Bais the greatest among alkaline earth metals and also greater than thatof Sn in the form of atom.

In FIG. 4, x-ray diffraction pattern of BaSnO₃, SrSnO₃, CaSnO₃ andMgSnO₃ in the first charged state are shown. In each pattern, a peakattributed to the crystal structure of each compound is observed andBaSnO₃ exhibits the strongest peak. As these patterns show, eachcompound keeps its primary crystal structure even in the charged state.Among them, BaSnO₃ is considered to be the most stable.

On the other hand, Table 2 shows that Sr_(0.1)Ba_(0.9)SnO₃,Sr_(0.3)Ba_(0.7)SnO₃ and Sr_(0.5)Ba_(0.5)SnO₃ have peculiarly largedischarge capacity. This indicates that Sr_(x)Ba_(1−x)SnO₃ in the rangeof 0.1≦x≦0.5 is especially excellent as the active material for thebattery.

EXAMPLE 2

In this example, various lithium composite compounds were prepared byintercalating a specified amount of lithium in MgSnO₃ of therepresentative negative electrode active material of the presentinvention and evaluated for their electrode characteristics.

First, electrodes were produced using the above active materials, whichwere then integrated in test cells in the same manner as in Example 1.Then, the lithium amount capable of intercalating in each of theelectrodes was estimated by regulating the quantity of electricityconsumed by cathode polarization and anode polarization. After tested,the cells were disassembled for quantitation of the lithium compositecompounds by ICP spectrometry. This analysis confirmed a coincidence ofthe composition with the estimated composition in each compositecompound.

Next, for evaluation of the cycle life characteristics of the batteriesapplied with the negative electrodes of the various lithium compositecompounds of Example 2, cylindrical batteries as used in Example 1 wereproduced and evaluated under the same conditions as in Example 1. Atthat time, the intercalated lithium amount in the negative electrodeactive materials was adjusted by the amount of active material used.

After evaluation, the batteries were disassembled similarly to removethe negative electrode. The lithium composite compound thus harvestedwas quantitated by ICP spectrometry, which confirmed the composition ofeach compound. The analytical results are shown in Table 3.

TABLE 3 Lithium complex Capacity Capacity compositions (mAh/g)maintenance rate (%) Li_(0.1)MgSnO₃ 200 75 Li_(0.5)MgSnO₃ 400 80LiMgSnO₃  550 80 Li₂MgSnO₃ 600 90 Li₃MgSnO₃ 620 90 Li₄MgSnO₃ 650 95Li₅MgSnO₃ 650 90 Li₆MgSnO₃ 670 95 Li₇MgSnO₃ 680 95 Li₈MgSnO₃ 670 90Li₉MgSnO₃ 640 90 Li₁₀MgSnO₃ 580 85 Li₁₁MgSnO₃ 200 23 Li₁₂MgSnO₃ 125 15

The lithium composite compounds represented by the composition formulaLi_(θ)MgSnO₃ were found to manifest excellent electrode characteristicsin a range of 1≦θ<10. In other words, in that range, those compoundsgrow no metallic lithium dendrites and show good reversibility with highdischarge capacity maintenance ratios.

When 10≦θ, poor cycle life characteristics were confirmed in thecomposite compounds. This may be because those compounds are prone toproduce inactive lithium due to too much intercalation of lithium,resulting in poor cycle life characteristics. If the lithium amount isregulated to 0<θ<1, those compounds fail to produce a sufficientcapacity for a battery upon operation of the battery due to smallamounts of utilizable lithium.

In this example, Mg was used as the group D element included in theoxides but compounds whose D elements are selected from other alkalineearth metals can also produce similar effects.

In the foregoing examples, cylindrical batteries were used, but thepresent invention is not limited to such battery configuration and canexert identical inventive effects when applied to coin-shaped, angularor flat secondary batteries.

The foregoing examples used LiMn_(1.8)Co_(0.1)O₄ as the positiveelectrode, but any other positive electrode active materials includingLiMn₂O₄, LiCoO₂, LiNiO₂ and the like which have reversible chargeabilityand dischargeability may be used to obtain similar effects.

EXAMPLE 3

In this example, the peculiarity of the case in which Sr_(x)Ba_(1−x)SnO₃(0.03≦x≦0.5) is used as the active material is studied.

In FIG. 5, x-ray diffraction patterns of a powder of BaSnO₃ andSr_(x)Ba_(1−x)SnO₃ having representative x values are shown. From thesepatterns, it is found that all of these compounds have the sameperovskite structure. That is, Sr atoms are substituted for Ba atoms inthe same place where Ba atoms were positioned.

In FIG. 5, in the range of 0.03≦x≦0.1, the peaks are shifted to thelower angle side than the peaks of BaSnO₃. This suggests that thespacing of lattice planes is enlarged in this range.

Since Sr has smaller ionic diameter than Ba, it is generally estimatedthat the spacing of lattice planes will become smaller if some Sr atomsare substituted for some Ba atoms. In FIG. 5, however, the spacing oflattice planes is increased in the range of 0.03≦x≦0.1, it is consideredthat a distortion is caused in the crystal.

On the other hand, in the range of 0.03≦x≦0.1, it is confirmed that thecapacity of Sr_(x)Ba_(1−x)SnO₃ increases to be twice as much as that ofBaSnO₃. As an example, the charge/discharge cycle characteristics ofSr_(0.1)Ba_(0.9)SnO₃ is shown in comparison with that of BaSnO₃, SrSnO₃,CaSnO₃ and MgSnO₃. This charge/discharge cycle characteristics weremeasured by using the similar test cell as in Example 1 and repeatingthe same cathode polarization and anode polarization. FIG. 6 shows thatthe test cell using Sr_(0.1)Ba_(0.9)SnO₃ can maintain the capacity ofaround 90% of the initial capacity even over 200 cycles.

Further, the density of electrode mixture comprising the compound as theactive material is important for the battery. That is because if thedensity of the electrode mixture is greater, the discharge capacity perunit volume becomes larger, thereby increasing the capacity of. thebattery. Therefore, the similar electrode. mixture as in Example 1 wasprepared using BaSnO₃ or Sr_(x)Ba_(1−x)SnO₃ having a representative Xvalue, pressed and molded the resultant to produce an electrode andmeasured the density of the electrode. The results are shown in Table 4.

TABLE 4 x value 0 0.03 0.05 0.1 0.3 0.5 1 Compound BaSnO₃Sr_(x)Ba_(1−x)SnO₃ SrSnO₃ Mixture density 3.2 3.5 3.5 3.6 3.2 3.0 2.4(g/cc)

Table 4 shows that the density of the electrode mixture is increased inthe range of 0.03≦x≦0.1 to obtain a battery with large capacity.

As discussed above, according to the present invention, a highlyreliable non-aqueous electrode secondary battery which is free ofdendrite-induced short-circuiting and affords a high energy density canbe obtained by an application of a negative electrode with a highcapacity and an exceptional cycle life.

Although the present invention has been described in terms of thepresently preferred embodiments, it is to be understood that suchdisclosure is not to be interpreted as limiting. Various alterations andmodifications will no doubt become apparent to those skilled in the artto which the present invention pertains, after having read the abovedisclosure. Accordingly, it is intended that the appended claims beinterpreted as covering all alterations and modifications as fall withinthe true spirit and scope of the invention.

What is claimed is:
 1. A non-aqueous electrolyte secondary batterycomprising a chargeable and dischargeable positive electrode, anon-aqueous electrolyte and a chargeable and dischargeable negativeelectrode, said negative electrode comprising a compound represented bythe formula  DSnO₃ wherein D is represented by the formulaSr_(x)Ba_(1−x) wherein 0.03≦x≦0.5.
 2. The non-aqueous electrolytesecondary battery in accordance with claim 1, wherein 0.1≦x≦0.5.